This invention relates generally to wear pads used between sliding metal parts and more particularly to wear pads used in extendable booms of material handling equipment. Wear pads, used in booms of material handling equipment, have typically been pieces of a material sandwiched between two plates of steel (in any orientation) which slide back and forth. The material in between the two plates has historically been something of a moderately low coefficient of friction.
As is well known, when two sliding metal boom sections move with respect to each other, while simultaneously carrying a load, a wear pad between the respective boom sections is subject to a dynamic compressive loading condition that is represented by a characteristic known in the art as "PV value". The PV value recognizes the simultaneous imposition on the wear pad of load (compression) and movement of the boom sections. As is well known, the PV value for wear pad material is calculated as:
compression in pounds per square inch times speed of movement in feet per minute, otherwise stated as PSI.times.FPM. The PV value so calculated is stated for a temperature, such as 73.degree. F. It is believed that subjecting a wear pad material to conditions beyond a maximum PV value or "PV Limit" will cause the wear pad material to break down or lose its ability to act as a lubricating surface, thereby causing damage to the sliding metal parts, possibly even binding up the parts completely.
When a boom designer calculates that the boom sections contemplated will exceed the wear pad's PV Limit, the designer must increase the overlap between boom sections or reduce the speed to get the PV value below the PV Limit specified for the wear pad material. Longer boom sections required for increased overlap are more expensive to build and maintain and generally increase the overall weight of the boom.
One type of wear pad material used for sliding boom weldments is a cast block of polyamide impregnated with oil. Appropriate wear pad material is sold under the product designation as "PA6G+OIL", supplied by Timco, Inc., P.O.Box 2059, Peekskill, N.Y., 10566. Such wear pad material has a PV Limit of 15,000, and the prior art teaches that such material should not be used on booms that exceed this limit.
The foregoing illustrates limitations known to exist in present booms for material handling equipment. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.